The alleged clinically observed association between the use of seclusion and the luteal phase of the menstrual cycle was studied in 113 consecutively admitted female psychiatric patients by means of a monitoring of oestrogen and progesterone levels, the use of seclusion, age, drug use, regularity of menses, and a history of hysterectomy. Twenty-one patients (18.6%) were in the luteal phase. During the study period, 14 patients (12.4%) had to be secluded. No significant difference in the frequency of seclusion between luteal patients and patients in the other phases could be demonstrated. The findings indicate that the luteal phase of the menstrual cycle is not associated with an increased occurrence of seclusion in female psychiatric patients.